Why is it valid to assume most chemical reactions are done under constant pressure, allowing the analyst to simply assume the heat of a chemical reaction is the change in enthalpy?

When the following equation is balanced using the smallest possible integers, what is the coefficent of oxygen gas? c7h16o(g) + o2(g) → co2(g) + h2o(g) -1 -5 -8 -16 -21

Agroup of students is studying convection currents. they fill two identical balloons with the same amount of helium. one balloon is placed in a freezer and the other in an area with warm air. after 10 minutes, the balloons are released from a height of 1 meter. which of the following do the students most likely observe? a. the balloons both rise. the cold balloon is larger than the warm balloon. b. the balloons rise at the same rate. both balloons are the same size. c. the warm balloon expands and rises. the cold balloon shrinks and sinks. d. the cold balloon expands and rises. the warm balloon shrinks and sinks.

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.